A valve train for gas exchange valves of an internal combustion engine, comprising a displaceable cam carrier with an endless groove extending over the periphery is disclosed in German Application No. DE 10 2007 061 353 A1. The cam carrier is non-rotatably, yet axially displaceably mounted on a camshaft and has a plurality of axially spaced cams having different cam protrusions, which are combined to form a cam group for the respective gas exchange valve. An engagement element engages continuously in the endless groove and implements a lift in the axial direction according to the course of the endless groove. The engagement element can be locked in specific positions, forcing the cam carrier to perform an axial movement and thus switch between the cams for driving a gas exchange valve.
However, the described system is very bulky, and therefore use is only possible in internal combustion engines having sufficient cylinder spacing. Furthermore, an actuator for locking the engagement element is required for each displaceable cam carrier and has to be activated at the correct moment in time.
German Application No. DE 10 2009 030 373 A1 describes a valve train for an internal combustion engine having variable-lift gas exchange valve actuation. The camshaft of the valve train comprises a carrier shaft and a cam assembly mounted non-rotatably, yet axially displaceably thereon. The cam assembly contains cam groups of directly adjacent cams having different protrusions. A link is provided at the end of the cam assembly to displace the cam assembly axially with respect to the carrier shaft by engagement of an actuation element. The cam assembly rotating with the carrier shaft is mounted in a camshaft bearing.
The adjustment link on the cam assembly is arranged outside the camshaft bearing, whereby further axial overall space is provided for each cam assembly. A separate actuation element is required for each adjustment direction. Two actuators are therefore used for each cylinder of the internal combustion engine. The complex controls and expensive components required, in conjunction with the spatial requirement constitutes a fundamental drawback.
A valve train having variable-lift gas exchange valve actuation is disclosed in German Application No. DE 10 2010 013 216 A1. The valve train consists of a camshaft, which is formed of a carrier shaft and cam assemblies mounted non-rotatably, yet axially displaceably thereon. A cam group of directly adjacent cams having different protrusions and an axial link are located on the cam assembly. The cam assembly is also provided with a journal and is mounted via said journal in a camshaft bearing. The journal and the adjacent axial link are overlapped by the camshaft bearing, and therefore the actuation element for displacing the cam assembly is positioned on the camshaft bearing and extends radially through the bearing point to engage in the axial link.
Since the axial link and, also, the journal are overlapped by the camshaft bearing, the bearing point has to be particularly wide so as to provide the necessary bearing area according to the bearing stresses. This leads to an increased requirement of overall space. The system can only be used in internal combustion engines having sufficiently large valve spacing or cylinder spacing. In addition, an actuation element for each cam assembly is also required in this system. This conflicts with the current development aim of relatively compact internal combustion engines.
A switchable valve train of an internal combustion engine is described in German Application No. DE 10 2007 022 145 A1. The camshaft of the valve train comprises a driveshaft having at least one cam assembly which is arranged non-rotatably, yet axially displaceably thereon and which has a group of axially adjacent cams. The cams have different cam protrusions. The cam assembly is supported radially in a camshaft bearing point, jointly with the driveshaft and with an additional bearing bush mounted co-axially on the cam assembly. The additional bearing bush is axially displaceable relative to the camshaft bearing point and the driveshaft, jointly with the cam assembly, wherein the bearing bush is secured against rotation in the camshaft bearing point. The cam assembly and the bearing bush are thus accordingly arranged co-axially one above the other on the driveshaft and are mounted in the camshaft bearing point. In the bearing bush, a dowel is positioned parallel to the axis of rotation of the driveshaft and performs an oscillating movement as a result of a guide formed in the peripheral cam assembly with axial lift. To initiate a switching operation, the dowel is temporarily fixed axially at the corresponding point by a fixing apparatus. The cam assembly is thus supported on the fixed dowel. An axial movement of the cam assembly is forced and a switching operation is initiated.
The complex controls and the associated expensive component constitute a drawback of this device for switching the lift. A separate fixing apparatus, which has to initiate a switching operation at the correct moment in time, must be provided for each switching device of a cam assembly. The dowel also leads to high stresses in the guide groove during the switching operation. Furthermore, the dowel is subject to a high level of wear since it has to be clamped for each switching operation.
U.S. Pat. No. 5,129,407 describes an adjustment device for a camshaft, with which the camshaft can be displaced axially. Different cams having different cam protrusions can be positioned for the actuation of the gas exchange valves by the displacement of the camshaft. To displace the camshaft, an axial link is provided, with which axial displacement in both directions is enabled. To this end, the link has two contours extending over the periphery in a mirror-symmetrical manner, wherein each of the two contours produces the lift necessary for shifting to shift the camshaft. A region in which the permanently engaged adjustment dowel can be positioned without making contact with one of the two peripheral contours is provided between the two contours. To initiate a switching operation, the adjustment dowel is displaced in the axial direction and fixed so as to come into contact with one of the two contours and initiate the axial shifting of the camshaft in the corresponding direction.
However, the camshafts and cams positioned thereon of modern internal combustion engines having a plurality of cylinders do not allow simultaneous adjustment of the cams and therefore also do not allow displacement of the entire camshaft, since there is no crossover of the cam base circles between the different cylinders. In addition, the link necessitates an increase in the overall length of the camshaft.